Some writers and speakers use misconstructions like "an increase by twofold," which they believe to mean the same thing as "a twofold increase." But these two things logically mean "an increase by 200% of the original amount" and "an increase to 200% of the original amount," respectively. This practice is not only ambiguous but grammatically poor; it misuses -fold by using it as a noun. -Fold takes no preposition.

In scientific contexts, "-fold" is sometimes appended to numerals (with the same sense), as in a 2010 paper by M.C. Stone et al., which mentions "10-fold up-regulation of the number of growing microtubules" in its abstract.